Those who had previous mental health problems - such as anxiety and depression - had the highest risk of experiencing poor mental health while they were expecting.

"This is the first national, population-based study that shows that the mother's previous mental health really matters," says Witt, assistant professor of population health sciences at the University of Wisconsin School of Medicine and Public Health.

The researchers examined data on 3,051 pregnant women ages 14 and older. The information was collected from pregnant women who took part in the Medical Expenditure Panel Survey between 1996 and 2006, a household survey cosponsored by the Agency for Healthcare Research and Quality and the National Center for Health Statistics.

"There is a need for greater focus on mental health screening for all women of reproductive age regardless of their pregnancy status, but especially for women before they become pregnant since it is such an important risk factor," Witt says.

Further, Witt says these findings suggest that continuity of care is vital to the health of all women.

"In an effort to move towards improved quality of care and outcomes for women, women's mental health needs to be monitored and treated appropriately over the lifecourse."